This invention relates to an FRP pipe having threaded ends at both ends, and in particular it relates to an FRP pipe with threaded ends which is suitable as casing and tubing for use in oil and gas wells.
Oil well casing or tubing for use in crude oil and natural gas wells are generally made of steel. However, in recent years, the environments of oil wells have become more severe ones which contain many corrosive materials such as CO.sub.2, H.sub.2 S, and Cl.sup.-. In addition, as crude oil resources decrease, the recovery method has become very popular in which CO.sub.2 and salt water are injected into a well. Under these conditions, the corrosion resistance of conventional carbon steel pipes has been found to be inadequate, and several ways to prevent corrosion are now commonly being taken, such as the use of inner surface coatings or corrosion-resistant steels containing large quantities of chromium, nickel, and the like.
However, inner surface coatings suffer from problems like the generation of pin holes on the end surfaces of pipes and damage to the coating during wire line operations, and as a result they are not very effective at increasing the life span of pipes in actual use. Corrosion-resistant steel pipes, on the other hand, contain large quantities of expensive alloying elements such as chromium and nickel, so the cost of these pipes is high.
In light of these circumstances, there is considerable interest in the use of fiberglass reinforced plastic (FRP) pipes, which are less expensive than the above-described corrosion-resistant steel pipes and have excellent corrosion resistance in the environments of oil wells. In the United States, the American Petroleum Institute (API) has already created specifications for FRP pipes for use in oil wells, and FRP pipes are beginning to be used in the oil field.
Oil well pipes are normally used for a certain period of time at one well, after which they are disassembled and reused at another well. Therefore, oil well pipes generally have threaded ends by means of which they can be joined to one another. This also is the case for FRP oil well pipes, which are equipped with threaded ends.
As is the case with steel oil well pipes, in order to increase the seal-tightness of the connections between FRP pipes, a lubricant is applied between the male and the female threads when joining them to one another. The lubricant is referred to as a "thread compound" and is a mixture of grease (mineral oil) and a solid lubricant such as graphite, MoS.sub.2, Pb, Cu, and Zr. After a period of use, however, the lubricant which was applied to the joint hardens and bonds to the thread surfaces because of the high-temperature environment in which oil well pipes are used. When the lubricant hardens and bonds to the FRP threads, it becomes difficult to separate the pipes from one another. Therefore, when disassembling the pipes, very high torque values may be applied in such case, which may damage the threads of the pipes, and in some cases may even break the pipes themselves.